Astronomers have been describing their most detailed view yet of the break-up of a comet.

Comet Linear came apart in July 2000, just a few days before its closest approach to the Sun. Many of its fragments were too small to see.

Data from Linear's demise has helped astronomers get a sense of the comet's component parts and where and how they may have come together.

"We think watching it come apart was a lot like seeing how the comet was put together in the first place played out in reverse," says Hal Weaver, of the Johns Hopkins University in Maryland in the United States.

Dirty snowball

Comets are mountain-sized balls of ice, dust and rock, which clumped together in the cold, distant regions from the Sun when the Solar System was forming.

"We have tended to think of cometary nuclei as being made up of roughly equal parts snow and meteoritic material, or 'dirty snowballs'," Weaver said.

However, observations with the Hubble Space Telescope (HST) suggests that Linear appears to have been mostly meteoritic material with some ice.

According to Weaver, that could mean that the comet was initially formed relatively close to the Sun, perhaps in the region where Jupiter now orbits, where not as much ice would have condensed upon the meteoritic material.

Using images from the HST, astronomers detected about 16 large pieces among the debris.

Images of Linear's scattered fragments suggest it split apart because of its relatively rapid rotation. "It's possible that this rotation, combined with Linear's approach to the Sun, could have contributed to the comet's demise," Weaver says.

Small fragments

But there was a puzzle. After the break-up, most of the comet appeared to go missing. Estimates of the mass of Linear's nucleus, based on the amount of icy material on its surface, ranged as high as 300 billion kilograms (660 bn pounds).

However, the estimated total mass in the largest fragments observed after the disintegration event was only about 3 billion kg (6.6 bn lbs).

Spectroscopic observations of the comet, before and after the break-up, made by ground-based telescopes, show that most of the comet's original mass is now in pieces between one millimetre (0.04 inches) and 50 metres (164 feet) in size and is simply too small to be seen from Earth.

Analysis of the light from the comet also suggests that carbon-containing compounds were relatively scarce within it, hinting once more that it formed in a warmer environment than most other comets.

Its warmer origins would have allowed volatile carbon-rich molecules to escape into space. If other comets turn out to be more like comet Linear, researchers may have to reconsider their theory that comets delivered the precursor molecules for life to Earth.

New Comets Found

Strangers in the Night

April 6, 2000 - NASA News

During an unplanned rendezvous,
the Ulysses spacecraft found itself gliding though the
immense tail of Comet Hyakutake, revealing that
comet tails may be much, much longer than previously
believed.

"The odds that Ulysses' flight path would intersect
the comet tail were probably less likely than
someone breaking the bank at Monte Carlo," said Dr.
Edward Smith of NASA' s Jet Propulsion Laboratory,
Pasadena, CA, the Ulysses project scientist and a co-
investigator for its magnetometer instrument. Before
the unexpected encounter, Ulysses was hundreds of
millions of miles, or kilometers, away from Comet
Hyakutake and far beyond the visible tail.

"This tail extends half a billion kilometers (more than 300 million miles). That's more
than three times the distance from the Earth to the Sun," said Dr. Nathan
Schwadron, of the University of Michigan in Ann Arbor, a member of one of two
Ulysses teams that made the discovery independently of one another. Findings
from both teams appear in the April 6 issue of the journal Nature.

Comet Hyakutake, one of the brightest comets of the 20th century, made a
dazzling nighttime appearance in the spring of 1996, when it made a close pass by
the Sun. While Ulysses was cruising through space studying the solar wind on May
1, 1996, its data suddenly went wild for a few hours. For example, the solar wind
seemed to almost disappear and was replaced by gases not normally found in the
solar wind, and the magnetic field in the solar wind was distorted. Since Ulysses
scientists were not looking for comets, they did not realize the significance of the
data right away.

Four years ago, the Great Comet of 1996, Comet
Hyakutake, inched across our northern sky during its
long orbit around the Sun.

The discovery was made quite by
accident, a bit like finding a needle in a
haystack when you weren't even
looking for a needle in the first place,"
said Dr. George Gloeckler of the
University of Maryland, principal
investigator of the Ulysses solar-wind
ion- composition spectrometer team.

The instrument studies the content and electrical charge of ionized gases. While
his team detected ions typically found in comets, the magnetometer team
observed magnetic field directional changes like those associated with comet tails.

Comets are of great interest, because they may be the frozen leftovers of the
birth of our solar system. They could hold clues to the formation of Earth and life,
since one theory holds that comets "seeded" Earth and other planets with the
building blocks of life.
Comets are made of dirty ice, and as they approach the Sun and
heat up, they emit gas and dust, forming gas and dust tails. The
gas slows the solar wind and the portion of the magnetic field
near the comet. The parts of the magnetic field farther from the
comet continue to travel rapidly past it. Magnetic fields can be
stretched like rubber bands. The magnetic field is draped around
the comet and stretches out behind it in a hairpin shape.

Gloeckler is lead author of the Nature paper on the ion findings,
along with Schwadron, and Drs. Lennard Fisk and Thomas
Zurbuchen, also of the University of Michigan, and Dr. Johannes
Geiss of the International Space Science Institute in Switzerland. The other Nature
article, on the Ulysses magnetometer findings, was authored by Jones and
Professor Andre Balogh of Imperial College and Dr. Timothy Horbury of Queen Mary
and Westfield College, London.

Jones at Imperial College looked more closely at the magnetic field data because of
the publication of the unusual 1996 solar wind event in the Journal of Geophysical
Research. It was authored by Dr. Peter Riley, formerly of Los Alamos National
Laboratory in New Mexico, and based on data from the Ulysses solar wind
instrument. Jones and Horbury saw that the data looked like a cometary tail, and
Jones searched until he found the tail's source -- Hyakutake. Gloeckler and his
colleagues noticed the event independently and realized it was cometary material.

Where did Comet Hyakutake come from? The
orbits of the Earth and this brightening comet are shown
in the above diagram. The blue disk is bounded by the
circular orbit of the Earth about the central Sun. The
comet's path, which is nearly a parabola, outlines the
green shape.

Ulysses, launched in 1990, is a joint
venture of NASA and the European
Space Agency (ESA). The spacecraft
studies the Sun from a high-latitude
orbit, mostly at right angles to the
plane of orbiting planets. Ulysses studies the Sun's magnetic fields, solar winds and
cosmic rays near the Sun's North and South Poles, away from the equator, where
Earth orbits. Ulysses has no camera, but its ten sophisticated instruments can
observe some phenomena not detectable by visible observations. Scientists now
know that sensitive instruments, like those found on Ulysses, can detect comet tail
particles that are not normally visible.

Ninety-two sungrazing comets discovered by SOHO appear to have come from the breakup
of a single gigantic comet more than 2000 years ago.

In just four years of operation, the Solar and Heliospheric Observatory (SOHO) spacecraft has
found102 comets, making it by far the most successful comet-hunter in history. Most of this
amazing number are suicidal comets that vaporize as they plunge into the solar atmosphere.

One hundred years ago Heinrich Kreutz in Kiel, Germany, realized that several comets seen buzzing
the Sun seemed to have a common origin, because they came from the same direction among the
stars. These comets are now called the Kreutz sungrazers. A whopping 92 of SOHO's 102 comet
discoveries belong to that class.

"SOHO is seeing fragments from the gradual breakup of a great comet, perhaps the one that the
Greek astronomer Ephorus saw in 372 BC," said Dr. Brian Marsden of the Center for Astrophysics in
Cambridge, MA. "Ephorus reported that the comet split in two. This fits with my calculation that two
comets on similar orbits revisited the Sun around AD 1100. They split again and again, producing the
sungrazer family, all still coming from the same direction."

The history of splitting gives clues to the strength of comets, which will be of practical importance if
ever a comet seems likely to hit the Earth. Also, the fragments seen as SOHO comets reveal the
internal composition of comets, freshly exposed, in contrast to the much-altered surfaces of objects
like Halley's Comet that have visited the Sun many times.

Earth battered through history by comets

Researchers say impacts caused global crises, mass extinctions

August 17, 1999 - Mercury News Washington Bureau - Washington

Recent scientific discoveries are
shedding new light on why great empires such as Egypt,
Babylon and Rome fell apart, giving way to the periodic
"dark ages" that punctuate human history.

At least five times during the last 6,000 years, major
environmental calamities undermined civilizations around
the world. Some researchers say these disasters
appear to be linked to collisions with comets or
fragments of comets like the one that broke apart and
smashed spectacularly into Jupiter five years ago this
summer.

The impacts, yielding many megatons of explosive
energy, produced vast clouds of smoke and dust that
circled the globe for years, dimming the sun, driving
down temperatures and sowing hunger, disease and
death.

The discoveries are changing the way scientists and
historians look at the past -- and offer a warning about
what might happen to our planet in centuries to come.

The last such global crisis occurred between AD 530
and 540 -- at the beginning of the Dark Ages in Europe
-- when Earth was pummeled by a swarm of cosmic
debris.

In a forthcoming book, "Catastrophe, the Day the Sun
Went Out,'' British historian David Keys describes a
two-year-long winter that began in AD 535. Trees from
California to Ireland to Siberia stopped growing. Crops
failed. Plague and famine decimated Italy, China and the
Middle East.

Keys quotes the writings of a sixth-century Syrian
bishop, John of Ephesus: "The sun became dark. .ð.ð.Each day it shone for about four hours and still this light
was only a feeble shadow.'' A contemporary Italian
historian, Flavius Cassiodorus, wrote: ``We marvel to
see no shadows of our bodies at noon. We have
summer without heat.'' And a contemporary Chinese
chronicler reported, ``Yellow dust rained like snow.''

Researchers say similar environmental calamities
occurred around 3200 B.C., 2300 B.C., 1628 B.C. and
1159 B.C. Each led to the collapse of urban societies in
widely scattered portions of the globe.

Destructive as they were, the natural disasters that
have plagued Earth since the dawn of human civilization
are but popguns compared with the truly titanic
catastrophes of prehistoric eras.

There have been at least five of these monster events,
each of which wiped out most of the creatures living at
the time, the fossil record shows.

The best known was a six-mile-wide meteor that
smashed into what is now the Gulf of Mexico 65 million
years ago. The collision wreathed the planet in clouds
of dust, poisoned the atmosphere and drove the
dinosaurs, then rulers of the Earth, into extinction.
Traces of the enormous crater, at least 100 miles
across, created by the impact were found in 1990.

Even that wasn't the biggest blow the Earth has
suffered. The mother of all extinctions, which wiped out
90 percent of living species, happened about 245 million
years ago. Paleontologists say other mass extinctions
occurred about 214 million, 360 million and 440 million
years ago.

Although the evidence is debated, a growing number of
researchers contend that most, if not all, of these
ecological disasters are connected to bombardments
from space.

``Recent evidence is converging on the conclusion that
mass extinctions coincided with comet or asteroid
impacts, and that periodic comet showers, triggered by
the solar system's motions through the Milky Way
galaxy, may provide a general theory to explain
impact-related mass extinctions,'' said Michael Rampino,
a geologist at New York University.

These disasters, while terrible for their victims, opened
the way for the survivors to flourish, diversify and -- for
humans -- take over the world.

These discoveries are lending weight to a revised
theory of evolution. Instead of proceeding gradually by
a series of tiny changes, as Charles Darwin proposed
140 years ago, life developed in a series of starts and
stops, biologists now believe. They call it "punctuated
evolution,'' periods of slow development interrupted by
wholesale extinctions and recoveries.

Evidence supporting this catastrophic theory of
evolution is accumulating from many sources:

Studies of oak and pine tree rings in Europe and
North America provide a year-by-year chronology of
good times and bad dating back 5,000 years. Extremely
narrow growth rings are testimony to environmental
setbacks that coincide with human catastrophes.

Ice cores recently pulled out of glaciers in Greenland
and Antarctica preserve a record of environmental
changes over the last 400,000 years.

Deep ocean drilling and surveys on land have
detected more than 150 impact craters -- like the
mile-wide Meteor Crater in Arizona -- demonstrating that
Earth has been the target of frequent bombardment
from space. Three or four craters are discovered each
year, and many more are thought to be buried
underground or in the sea.

NASA and the Air Force are searching for comets
and asteroids that might be on a collision course with
our planet. Fortunately, nothing of a dangerous size --
arbitrarily defined as more than a kilometer (0.6 miles) in
diameter -- has been spotted heading our way for at
least a century. But astronomers say a major impact is
inevitable.